Hot-melt adhesives are solid at an ordinary temperature. In use, they are thermally melted, applied to substrates and set by being cooled to cause adhesion, whereby they exhibit bond strength.
These hot-melt adhesives have numerous advantages of having 100% solids content and being substantially free of fire hazards, capable of setting in a shorter period of time and easy to handle. Because of these advantages and with rapid spread of applicators, the hot-melt adhesives have found prevalent use in various fields such as bookbinding, packaging, woodbonding and the like.
Generally these hot-melt adhesives comprise: a base polymer such as ethylene-vinyl acetate copolymer (EVA), ethylene-acrylate copolymer and like ethylene copolymers, polyethylene, thermoplastic rubber, polyamide, polyurethane, polyester and like thermoplastic high-molecular-weight polymers, etc.; a tackifying resin; and a viscosity-controlling agent such as various kinds of waxes. Widely used as the base polymer are ethylene copolymers such as EVA, ethylene-acrylate and the like because they give adhesives favorable in respect of flexibility, thermostability, costs, etc. Useful tackifying resins include natural resins such as rosin resin and terpene resin and petroleum resins.
Among these tackifying resins, common rosin resins have been predominantly used because they give adhesives high in adhesive property and cold resistance. But they can not provide adhesives satisfactory in thermostability and setting time. The rosin resin has been disproportionated or hydrogenated or mixed with an expensive antioxidant to overcome the defect of low thermostability among the defects of rosin resin. However, the disproportionation or hydrogenation results in the production of adhesives as poor in setting property as those prepared from common rosin resins, although having an improved thermostability. Further a hydrogenated resin involves economical disadvantage, hence unfit for practical use. The use of antioxidant encounters other drawbacks of manufacturing adhesives unacceptable in coloration and odor, hence undesirable.
Terpene resins, chiefly terpene-phenol resins, give adhesives excellent in thermostability and setting time, but unsatisfactory in heat resistance, cold resistance and odor.
Use has been proposed and made of petroleum resins prepared by polymerization of inexpensive petroleum fractions as tackifing resins in place of rosin resins and terpene resins. While relatively good in setting property, the adhesives prepared from petroleum resins are inferior to those from the natural resins in compatibility, thermostability, adhesive property, cold resistance, etc.
As stated above, even if incorporating any of conventional tackifying resins, hot-melt adhesives are poor in at least one of thermostability, compatibility, adhesive property, setting time and cold resistance. Hot-melt adhesives satisfactory in all of these characteristics have not been developed yet.
Directing attention to stabilized rosin esters relatively superior in these characteristics to other conventional tackifying resins, we conducted extensive research to develop novel and useful hot-melt adhesives free of the foregoing drawbacks. As a result, we unexpectedly found that a specific type of stabilized rosin ester used as the tackifying resin gives hot-melt adhesives free of all the foregoing drawbacks and particularly the poor setting property in which known stabilized rosin esters have the most serious drawback. Based on this novel finding, we have accomplished this invention.